Hypomagnesemia and hypermagnesemia could have serious implications and possibly lead to progress from a mild form to a severe outcome of Covid-19. Susceptibility of subjects with low magnesium status to develop and enhance this infection is possible. There is little data on the magnesium status of patients with Covid-19 with different degrees of severity. This study was conducted to evaluate prevalence of dysmagnesemia in a prospective Covid-19 cohort study according to the severity of the clinical manifestations and to identify factors associated. Serum magnesium was measured in 300 of 549 patients admitted to the hospital due to severe Covid-19. According to the WHO guidelines, patients were classified as moderate, severe, or critical. 48% patients had a magnesemia below 0.75 mmol/L (defined as magnesium deficiency) including 13% with a marked hypomagnesemia (<0.65 mmol/L). 9.6% had values equal to or higher than 0.95 mmol/L. Serum magnesium concentrations were significantly lower in female than in male (0.73 +/- 0.12 vs 0.80 +/- 0.13 mmol/L), whereas the sex ratio M/F was higher in severe and critical form (p<0.001). In a bivariate analysis, the risk of magnesium deficiency was significantly and negatively associated with infection severity (p<0.001), sex ratio (M/F, p<0.001), oxygenotherapy (p<0.001), stay in critical care unit (p=0.028), and positively with nephropathy (p=0.026). Logistic regression analysis revealed that the strongest predictors of magnesium deficiency were female sex (OR=2.67, p<0.001) and nephropathy (OR=2.12, p=0.032) and after exclusion of sex ratio, the severity of infection (OR=0.46, p=0.04 and OR=0.39 p=0.01), for critical and moderate forms, respectively. This transversal study reveals a high prevalence of hypomagnesemia in hospitalized patients for Covid-19, while high-level serum magnesium concentration was more prevalent in critical form.